The function of integrated circuits (ICs) can be disturbed by transient faults, also known as Single Even Upsets (SEUs) or soft errors. These are mainly due to secondary atmospheric neutrons caused by cosmic radiation. Transient faults flip the state of a single register, until the state of this register is re-written.
Another fault class is stuck-at faults. These tie a node permanently to 0 or 1. They may be caused by metal particles that short a node to a ground or a supply wire. Most ICs with a stuck-at fault are screened out in the production test, but in some ICs the fault becomes active only after some time of operation in the field.
For an IC in safety-critical systems, such as automotive systems, a malfunction caused by a stuck-at or a transient fault might endanger a person's safety, so measures are required to detect and mitigate possible impairments by such faults.
Automotive standards like ISO 26262 require manufacturers to prove that a certain percentage of safety-critical faults are detected and mitigated. This percentage is called “Diagnostic Coverage” and must be at least 90% to 99%, depending on the safety level of the device (ASIL, Automotive Safety Integrity Level).
The conventional method of detecting stuck-at faults uses a Scan-BIST (Built-In Self-Test). This is a scan test where all necessary logic to create and compare scan patterns is implemented on-chip. As Scan-BIST overwrites all registers values, they need to be restored after each BIST run. Moreover, the area needed for the additional on-chip hardware is relatively large.